1. Field
The present disclosure relates to tire treads for vehicles and notably to the geometries of the grooves and slits provided in these treads.
2. Description of Related Art
There is a known way of providing a tread of a tire intended to be fitted on a vehicle with a plurality of circumferentially and transversely orientated grooves, these grooves delimiting a plurality of blocks forming a tread pattern. This tread comprises a rolling surface adapted to come into contact with a road while the tire is running. Each block comprises a contact face forming part of the rolling surface of the tread and lateral faces delimiting the grooves. These lateral faces cut the contact face at edges.
In the context of certain uses or applications, there is a known way of increasing the adhesion of these treads by providing at least some of these blocks with slits whose depth may be substantially identical to the depth of the grooves or, in some applications, may be chosen with a depth which is less than the depth of the grooves. Thus the tread comprises a plurality of edges of material, these edges being formed by the intersection of the grooves and slits with the rolling surface of the tread. Because of the presence of slits, it is possible to increase the number of edges without excessively reducing the rigidity of the tread and without excessively increasing the volume of voids in this tread.
There is also a known method, described in EP 1074405 and WO2002102611, of forming a chamfered part on the walls delimiting the transversely orientated grooves and slits, in the vicinity of the contact faces of the blocks of the tread. In the present application, the term “chamfer” denotes a flat part inclined with respect to the lateral face, which it extends up to the contact face, and also a curved part that may be tangentially connected to one or other of said faces, or to both faces. A chamfer is defined by a height, measured on the lateral face, and a width, the latter dimension being measured on the contact face. These chamfers may modify the edges formed by the grooves and slits.
These chamfers are useful in that they improve the mechanics of the contact of each block with the road, at least in the new state, and notably on roads covered with water in rainy weather. The chamfers modify the way in which the edges of the contact faces of the blocks of the tread pattern come into contact with the road during running in a straight line. Although these chamfers do not remain as permanent features during the wearing of the tread, they have been found to be particularly useful when the tire is new, that is to say when the tire has not yet been used for running. It has also been found that these chamfers could be self-maintaining or even self-regenerating, notably in emergency braking.
A substantial improvement of braking performance has been found in tires provided with treads having chamfers.
These chamfer arrangements provide high performance in straight-line running, since there is little change in the shape of the footprint, and this shape has substantially the same effect on all the blocks of the tread, regardless of whether they are located in the median part, on the edges, or on the intermediate parts of the tread.
In use, however, it has been found that the chamfers formed on the transverse edges of the tread may create noise when running, notably when the tread is stressed in a cornering configuration.
It is known that a tire mounted on a vehicle has a footprint substantially centred on the trace of its equatorial plane when the vehicle runs in a straight line. However, during a cornering manoeuvre, it is known that there is a load transfer to the tires located on the side of the vehicle on the outside of the corner. More precisely, the load is transferred to the axially outer parts of these tires. This load transfer is manifested by a change in the shape of the tire footprints.
However good the performance of tread patterns having chamfers on the transversely orientated cut-outs may be, an emission of noise is still found during cornering, posing a problem which requires an appropriate solution.